Catalytic depolymerization of polymers containing electrophilic linkages using nucleophilic reagents

ABSTRACT

A method is provided for carrying out depolymerization of a polymer containing electrophilic linkages in the presence of a catalyst and a nucleophilic reagent, wherein production of undesirable byproducts resulting from polymer degradation is minimized. The reaction can be carried out at a temperature of 80° C. or less, and generally involves the use of an organic, nonmetallic catalyst, thereby ensuring that the depolymerization product(s) are substantially free of metal contaminants. In an exemplary depolymerization method, the catalyst is a carbene compound such as an N-heterocyclic carbene, or is a precursor to a carbene compound. The method provides an important alternative to current recycling techniques such as those used in the degradation of polyesters, polyamides, and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/355,554, which is a divisional of U.S. patent application Ser. No.10/330,853 filed Dec. 26, 2002. The disclosures of the aforementionedapplications are incorporated by reference in their entireties.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made in part with Government support under a grantfrom the National Science Foundation (Cooperative Agreement No.DMR-980677). Accordingly, the Government may have certain rights to thisinvention.

TECHNICAL FIELD

This invention relates generally to the depolymerization of polymers,and, more particularly relates to an organocatalytic method fordepolymerizing polymers using nucleophilic reagents. The invention isapplicable in numerous fields, including industrial chemistry andchemical waste disposal, plastics recycling, and manufacturing processesrequiring a simple and convenient method for the degradation ofpolymers.

BACKGROUND OF THE INVENTION

Technological advances of all kinds continue to present many complexecological issues. Consequently, waste management and pollutionprevention are two very significant challenges of the 21^(st) century.The overwhelming quantity of plastic refuse has significantlycontributed to the critical shortage of landfill space faced by manycommunities. For example, poly(ethylene terephthalate)(poly(oxy-1,2-ethanediyl-oxycarbonyl-1,4-diphenylenecarbonyl); “PET”), awidely used engineering thermoplastic for carpeting, clothing, tirecords, soda bottles and other containers, film, automotive applications,electronics, displays, etc. will contribute more than 1 billion poundsof waste to land-fills in 2002 alone. The worldwide production of PEThas been growing at an annual rate of 10% per year, and with theincrease in use in electronic and automotive applications, this rate isexpected to increase significantly to 15% per year. Interestingly, theprecursor monomers represent only about 2% of the petrochemical stream.Moreover, the proliferation of the use of organic solvents, halogenatedsolvents, water, and energy consumption in addressing the need torecycle commodity polymers such as PET and other polyesters has createdthe need for environmentally responsible and energy efficient recyclingprocesses. See Nadkarni (1999) International Fiber Journal 14(3).

Significant effort has been invested in researching recycling strategiesfor PET, and these efforts have produced three commercial options;mechanical, chemical and energy recycling. Energy recycling simply burnsthe plastic for its calorific content. Mechanical recycling, the mostwidespread approach, involves grinding the polymer to powder, which isthen mixed with “virgin” PET. See Mancini et al. (1999) MaterialsResearch 2(1):33-38. Many chemical companies use this process in orderto recycle PET at the rate of approximately 50,000 tons/year per plant.In Europe, all new packaging materials as of 2002 must contain 15%recycled material. However, it has been demonstrated that successiverecycling steps cause significant polymer degradation, in turn resultingin a loss of desirable mechanical properties. Recycling using chemicaldegradation involves a process that depolymerizes a polymer to startingmaterial, or at least to relative short oligomeric components. Clearly,this process is most desirable, but is the most difficult to controlsince elevated temperature and pressure are required along with acatalyst composed of a strong base, or an organometallic complex such asan organic titanate. See Sako et al. (1997) Proc. of the 4^(th) Int'lSymposium on Supercritical Fluids, pp. 107-110. The use of such acatalyst results in significant quantities of undesirable byproducts,and materials processed by these methods are thus generally unsuitablefor use in medical materials or food packaging, limiting their utility.Moreover, the energy required to effect depolymerization essentiallyeliminates sustainability arguments.

Accordingly, there is a need in the art for an improved depolymerizationmethod. Ideally, such a method would not involve extreme reactionconditions, use of metallic catalysts, or a process that results insignificant quantities of potentially problematic by-products.

SUMMARY OF THE INVENTION

The invention is directed to the aforementioned need in the art, and, assuch, provides an efficient catalytic depolymerization reaction thatemploys mild conditions, wherein production of undesirable byproductsresulting from polymer degradation is minimized. The reaction can becarried out at temperatures of at most 80° C., and, because anonmetallic catalyst is preferably employed, the depolymerizationproducts, in a preferred embodiment, are substantially free of metalcontaminants. With many of the carbene catalysts disclosed herein, thedepolymerization reaction can be carried out at a temperature of at most60° C. or even 30° C. or lower, i.e., at room temperature.

More specifically, in one aspect of the invention, a method is providedfor depolymerizing a polymer having a backbone containing electrophiliclinkages, wherein the method involves contacting the polymer with anucleophilic reagent and a catalyst at a temperature of less than 80° C.An important application of this method is in the depolymerization ofpolyesters, including homopolymeric polyesters (in which all of theelectrophilic linkages are ester linkages) and polyester copolymers (inwhich a fraction of the electrophilic linkages are ester linkages andthe remainder of the electrophilic linkages are other than esterlinkages).

In a related aspect of the invention, a method is provided fordepolymerizing a polymer having a backbone containing electrophiliclinkages, wherein the method involves contacting the polymer with anucleophilic reagent and a catalyst that yields depolymerizationproducts that are substantially free of metal contaminants. The polymermay be, for example, a polyester, a polycarbonate, a polyurethane, or arelated polymer, in either homopolymeric or copolymeric form, asindicated above. In this embodiment, in order to provide reactionproducts that are substantially free of contamination by metals andmetal-containing compounds, the catalyst used is a purely organic,nonmetallic catalyst.

Preferred catalysts herein are carbene compounds, which act asnucleophilic catalysts, as well as precursors to carbene compounds, aswill be discussed infra. As is well understood in the art, carbenes areelectronically neutral compounds containing a divalent carbon atom withonly six electrons in its valence shell. Carbenes include, by way ofexample, cyclic diaminocarbenes, imidazol-2-ylidenes (e.g.,1,3-dimesityl-imidazol-2-ylidene and 1,3-dimesityl-4,5dihydroimidazol-2-ylidene), 1,2,4-triazol-3-ylidenes, and1,3-thiazol-2-ylidenes; see Bourissou et al. (2000) Chem. Rev.100:39-91.

Since the initial description of the synthesis, isolation, andcharacterization of stable carbenes by Arduengo (Arduengo et al. (1991)J. Am. Chem. Soc. 113:361; Arduengo et al. (1992) J. Am. Chem. Soc.114:5530), the exploration of their chemical reactivity has become amajor area of research. See, e.g., Arduengo et al. (1999) Acc. Chem.Res. 32:913; Bourissou et al. (2000), supra; and Brode (1995) Angew.Chem. Int. Ed. Eng. 34:1021. Although carbenes have now been extensivelyinvestigated, and have in fact been established as useful in manysynthetically important reactions, there has been no disclosure orsuggestion to use carbenes as catalysts in nucleophilic depolymerizationreactions, i.e., reactions in which a polymer containing electrophiliclinkages is depolymerized with a nucleophilic reagent in the presence ofa carbene catalyst.

Suitable catalysts for use herein thus include heteroatom-stabilizedcarbenes or precursors to such carbenes. The heteroatom-stabilizedcarbenes have the structure of formula (I)

wherein:

E¹ and E² are independently selected from N, NR^(E), O, P, PR^(E), andS, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and y areindependently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than O or S, then E¹ and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms;

R¹ and R² are independently selected from branched C₃-C₃₀ hydrocarbyl,substituted branched C₃-C₃₀ hydrocarbyl, heteroatom-containing branchedC₄-C₃₀ hydrocarbyl, substituted heteroatom-containing branched C₄-C₃₀hydrocarbyl, cyclic C₅-C₃₀ hydrocarbyl, substituted cyclic C₅-C₃₀hydrocarbyl, heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, andsubstituted heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl;

L¹ and L² are linkers containing 1 to 6 spacer atoms, and areindependently selected from heteroatoms, substituted heteroatoms,hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, and substituted heteroatom-containing hydrocarbylene;and

m and n are independently zero or 1, such that L¹ and L² are optional.

Certain carbene catalysts of formula (I) are novel chemical compoundsand are claimed as such herein. These novel carbenes are those wherein aheteroatom is directly bound to E¹ and/or E², and include, solely by wayof example, carbenes of formula (I) wherein E¹ is NR^(E) and R^(E) is aheteroalkyl or heteroaryl group such as an alkoxy, alkylthio, aryloxy,arylthio, aralkoxy, or aralkylthio moiety.

Carbene precursors suitable as catalysts herein include tri-substitutedmethanes having the structure of formula (PI), metal adducts having thestructure of formula (PII), and tetrasubstituted olefins having thestructure (PIII)

wherein, in formulae (PI) and (PII):

E¹ and E² are independently selected from N, NR^(E), O, P, PR^(E), andS, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and y areindependently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than O or S, then E¹ and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms;

R¹ and R² are independently selected from branched C₃-C₃₀ hydrocarbyl,substituted branched C₃-C₃₀ hydrocarbyl, heteroatom-containing branchedC₄-C₃₀ hydrocarbyl, substituted heteroatom-containing branched C₄-C₃₀hydrocarbyl, cyclic C₅-C₃₀ hydrocarbyl, substituted cyclic C₅-C₃₀hydrocarbyl, heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, andsubstituted heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl;

L¹ and L² are linkers containing 1 to 6 spacer atoms, and areindependently selected from heteroatoms, substituted heteroatoms,hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, and substituted heteroatom-containing hydrocarbylene;

m and n are independently zero or 1;

R⁷ is selected from alkyl, heteroalkyl, aryl, heteroaryl, aralkyl, orheteroaralkyl, substituted with at least one electron-withdrawingsubstituent;

M is a metal;

Ln is a ligand; and

j is the number of ligands bound to M.

In compounds of formula (PIII), the substituents are as follows:

E³ and E⁴ are defined as for E¹ and E²;

v and w are defined as for x and y;

R⁸ and R⁹ are defined as for R¹ and R²;

L³ and L⁴ are defined as for L¹ and L²; and

h and k are defined as for m and n.

The carbene precursors may be in the form of a salt, in which case theprecursor is positively charged and associated with an anioniccounterion, such as a halide ion (I, Br, Cl), a hexafluorophosphateanion, or the like.

Novel carbene precursors herein include compounds of formula (PI), thosecompounds of formula (PII) in which a heteroatom is directly bound to E¹and/or E², and those compounds of formula (PIII) in which a heteroatomis directly bound to at least one of E¹, E², E³, and E⁴, and may be inthe form of a salt as noted above.

Ideally, the carbene catalyst used in conjunction with the presentdepolymerization reaction is an N-heterocyclic carbene having thestructure of formula (II)

wherein:

R¹, R², L¹, L², m, and n are as defined above; and

L is a hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, or substituted heteroatom-containing hydrocarbylenelinker, wherein two or more substituents on adjacent atoms within L maybe linked to form an additional cyclic group.

As alluded to above, one important application of the present inventionis in the recycling of polyesters, including, by way of illustration andnot limitation: PET; poly (butylene terephthalate) (PBT); poly(alkyleneadipate)s and their copolymers; and poly(ε-caprolactone). Themethodology of the invention provides an efficient means to degrade suchpolymers into their component monomers and/or relatively shortoligomeric fragments without need for extreme reaction conditions ormetallic catalysts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the organocatalytic depolymerization of PET in thepresence of excess methanol using N-heterocyclic carbene catalyst, asevaluated in Example 7.

FIG. 2 illustrates the organocatalytic depolymerization of PET in thepresence of ethylene glycol using N-heterocyclic carbene catalyst, asevaluated in Example 6.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise indicated, this invention is not limited to specificpolymers, carbene catalysts, nucleophilic reagents, or depolymerizationconditions. The terminology used herein is for the purpose of describingparticular embodiments only and is not intended to be limiting.

As used in the specification and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Thus, for example, reference to “a polymer”encompasses a combination or mixture of different polymers as well as asingle polymer, reference to “a catalyst” encompasses both a singlecatalyst as well as two or more catalysts used in combination, and thelike.

In this specification and in the claims that follow, reference will bemade to a number of terms, which shall be defined to have the followingmeanings:

As used herein, the phrase “having the formula” or “having thestructure” is not intended to be limiting and is used in the same waythat the term “comprising” is commonly used.

The term “alkyl” as used herein refers to a linear, branched, or cyclicsaturated hydrocarbon group typically although not necessarilycontaining 1 to about 20 carbon atoms, preferably 1 to about 12 carbonatoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl,t-butyl, octyl, decyl, and the like, as well as cycloalkyl groups suchas cyclopentyl, cyclohexyl and the like. Generally, although again notnecessarily, alkyl groups herein contain 1 to about 12 carbon atoms. Theterm “lower alkyl” intends an alkyl group of 1 to 6 carbon atoms, andthe specific term “cycloalkyl” intends a cyclic alkyl group, typicallyhaving 4 to 8, preferably 5 to 7, carbon atoms. The term “substitutedalkyl” refers to alkyl substituted with one or more substituent groups,and the terms “heteroatom-containing alkyl” and “heteroalkyl” refer toalkyl in which at least one carbon atom is replaced with a heteroatom.If not otherwise indicated, the terms “alkyl” and “lower alkyl” includelinear, branched, cyclic, unsubstituted, substituted, and/orheteroatom-containing alkyl and lower alkyl, respectively.

The term “alkylene” as used herein refers to a difunctional linear,branched, or cyclic alkyl group, where “alkyl” is as defined above.

The term “alkenyl” as used herein refers to a linear, branched, orcyclic hydrocarbon group of 2 to about 20 carbon atoms containing atleast one double bond, such as ethenyl, n-propenyl, isopropenyl,n-butenyl, isobutenyl, octenyl, decenyl, tetradecenyl, hexadecenyl,eicosenyl, tetracosenyl, and the like. Preferred alkenyl groups hereincontain 2 to about 12 carbon atoms. The term “lower alkenyl” intends analkenyl group of 2 to 6 carbon atoms, and the specific term“cycloalkenyl” intends a cyclic alkenyl group, preferably having 5 to 8carbon atoms. The term “substituted alkenyl” refers to alkenylsubstituted with one or more substituent groups, and the terms“heteroatom-containing alkenyl” and “heteroalkenyl” refer to alkenyl inwhich at least one carbon atom is replaced with a heteroatom. If nototherwise indicated, the terms “alkenyl” and “lower alkenyl” includelinear, branched, cyclic, unsubstituted, substituted, and/orheteroatom-containing alkenyl and lower alkenyl, respectively.

The term “alkenylene” as used herein refers to a difunctional linear,branched, or cyclic alkenyl group, where “alkenyl” is as defined above.

The term “alkoxy” as used herein refers to a group —O-alkyl wherein“alkyl” is as defined above, and the term “alkylthio” as used hereinrefers to a group —S-alkyl wherein “alkyl is as defined above.

The term “aryl” as used herein, and unless otherwise specified, refersto an aromatic substituent containing a single aromatic ring or multiplearomatic rings that are fused together, directly linked, or indirectlylinked (such that the different aromatic rings are bound to a commongroup such as a methylene or ethylene moiety). Preferred aryl groupscontain 5 to 20 carbon atoms and either one aromatic ring or 2 to 4fused or linked aromatic rings, e.g., phenyl, naphthyl, biphenyl, andthe like, with more preferred aryl groups containing 1 to 3 aromaticrings, and particularly preferred aryl groups containing 1 or 2 aromaticrings and 5 to 14 carbon atoms. “Substituted aryl” refers to an arylmoiety substituted with one or more substituent groups, and the terms“heteroatom-containing aryl” and “heteroaryl” refer to aryl in which atleast one carbon atom is replaced with a heteroatom. Unless otherwiseindicated, the terms “aromatic,” “aryl,” and “arylene” includeheteroaromatic, substituted aromatic, and substituted heteroaromaticspecies.

The term “aryloxy” refers to a group —O-aryl wherein “aryl” is asdefined above.

The term “alkaryl” refers to an aryl group with at least one andtypically 1 to 6 alkyl, preferably 1 to 3, alkyl substituents, and theterm “aralkyl” refers to an alkyl group with an aryl substituent,wherein “aryl” and “alkyl” are as defined above. Alkaryl groups include,for example, p-methylphenyl, 2,4-dimethylphenyl, 2,4,6-trimethylphenyl,and the like. The term “aralkyl” refers to an alkyl group substitutedwith an aryl moiety, wherein “alkyl” and “aryl” are as defined above.

The term “alkaryloxy” refers to a group —O—R wherein R is alkaryl, theterm “alkarylthio” refers to a group —S—R wherein R is alkaryl, the termaralkoxy refers to a group —O—R wherein R is aralkyl, the term“aralkylthio” refers to a group —S—R wherein R is aralkyl.

The terms “halo,” “halide,” and “halogen” are used in the conventionalsense to refer to a chloro, bromo, fluoro, or iodo substituent. Theterms “haloalkyl,” “haloalkenyl,” and “haloalkynyl” (or “halogenatedalkyl,” “halogenated alkenyl,” and “halogenated alkynyl”) refer to analkyl, alkenyl, or alkynyl group, respectively, in which at least one ofthe hydrogen atoms in the group has been replaced with a halogen atom.

“Hydrocarbyl” refers to univalent hydrocarbyl radicals containing 1 toabout 30 carbon atoms, preferably 1 to about 20 carbon atoms, morepreferably 1 to about 12 carbon atoms, including linear, branched,cyclic, saturated, and unsaturated species, such as alkyl groups,alkenyl groups, aryl groups, alkaryl groups, and the like. The term“lower hydrocarbyl” intends a hydrocarbyl group of 1 to 6 carbon atoms,and the term “hydrocarbylene” intends a divalent hydrocarbyl moietycontaining 1 to about 30 carbon atoms, preferably 1 to about 20 carbonatoms, most preferably 1 to about 12 carbon atoms, including linear,branched, cyclic, saturated and unsaturated species. The term “lowerhydrocarbylene” intends a hydrocarbylene group of 1 to 6 carbon atoms.Unless otherwise indicated, the terms “hydrocarbyl” and “hydrocarbylene”are to be interpreted as including substituted and/orheteroatom-containing hydrocarbyl and hydrocarbylene moieties,respectively.

The term “heteroatom-containing” as in a “heteroatom-containing alkylgroup” (also termed a “heteroalkyl” group) or a “heteroatom-containingaryl group” (also termed a “heteroaryl” group) refers to a molecule,linkage, or substituent in which one or more carbon atoms are replacedwith an atom other than carbon, e.g., nitrogen, oxygen, sulfur,phosphorus or silicon, typically nitrogen, oxygen or sulfur. Similarly,the term “heteroalkyl” refers to an alkyl substituent that isheteroatom-containing, the term “heterocyclic” refers to a cyclicsubstituent that is heteroatom-containing, the terms “heteroaryl” andheteroaromatic” respectively refer to “aryl” and “aromatic” substituentsthat are heteroatom-containing, and the like. Examples of heteroalkylgroups include alkoxyaryl, alkylsulfanyl-substituted alkyl, N-alkylatedamino alkyl, and the like. Examples of heteroaryl substituents includepyrrolyl, pyrrolidinyl, pyridinyl, quinolinyl, indolyl, pyrimidinyl,imidazolyl, 1,2,4-triazolyl, tetrazolyl, etc., and examples ofheteroatom-containing alicyclic groups are pyrrolidino, morpholino,piperazino, piperidino, etc. It should be noted that a “heterocyclic”group or compound may or may not be aromatic, and further that“heterocycles” may be monocyclic, bicyclic, or polycyclic as describedabove with respect to the term “aryl.”

By “substituted” as in “substituted hydrocarbyl,” “substituted alkyl,”“substituted aryl,” and the like, as alluded to in some of theaforementioned definitions, is meant that in the hydrocarbyl, alkyl,aryl, or other moiety, at least one hydrogen atom bound to a carbon (orother) atom is replaced with a non-hydrogen substituent. Examples ofsuch substituents include, without limitation, functional groups such ashalide, hydroxyl, sulfhydryl, C₁-C₂₀ alkoxy, C₅-C₂₀ aryloxy, C₂-C₂₀ acyl(including C₂-C₂₀ alkylcarbonyl (—CO-alkyl) and C₆-C₂₀ arylcarbonyl(—CO-aryl)), acyloxy (—O-acyl), C₂-C₂₀ alkoxycarbonyl (—(CO)—O-alkyl),C₆-C₂₀ aryloxycarbonyl (—(CO)—O-aryl), halocarbonyl (—CO)—X where X ishalo), C₂-C₂₀ alkyl-carbonato (—O—(CO)—O-alkyl), C₆-C₂₀ arylcarbonato(—O—(CO)—O-aryl), carboxy (—COOH), carboxylato (—COO—), carbamoyl(—(CO)—NH₂), mono-(C₁-C₂₀ alkyl)-substituted carbamoyl (—(CO)—NH(C₁-C₂₀alkyl)), di-(C₁-C₂₀ alkyl)-substituted carbamoyl —(CO)—N(C₁-C₂₀alkyl)₂), mono-substituted arylcarbamoyl (—(CO)—NH-aryl), thiocarbamoyl(—(CS)—NH₂), carbamido (—NH—(CO)—NH₂), cyano(—C≡N), cyanato (—O—C≡N),formyl (—(CO)—H), thioformyl (—(CS)—H), amino (—NH₂), mono- anddi-(C₁-C₂₀ alkyl)-substituted amino, mono- and di-(C₅-C₂₀aryl)-substituted amino, C₂-C₂₀ alkylamido (—NH—(CO)-alkyl), C₆-C₂₀arylamido (—NH—(CO)-aryl), imino (—CR═NH where R=hydrogen, C₁-C₂₀ alkyl,C₅-C₂₀ aryl, C₆-C₂₄ alkaryl, C₆-C₂₄ aralkyl, etc.), alkylimino(—CR═N(alkyl), where R=hydrogen, alkyl, aryl, alkaryl, etc.), arylimino(—CR═N(aryl), where R=hydrogen, alkyl, aryl, alkaryl, etc.), nitro(—NO₂), nitroso (—NO), sulfo (—SO₂—OH), sulfonato (—SO₂—O⁻), C₁-C₂₀alkylsulfanyl (—S-alkyl; also termed “alkylthio”), arylsulfanyl(—S-aryl; also termed “arylthio”), C₁-C₂₀ alkylsulfinyl (—(SO)-alkyl),C₅-C₂₀ arylsulfinyl (—(SO)-aryl), C₁-C₂₀ alkylsulfonyl (—SO₂-alkyl),C₅-C₂₀ arylsulfonyl (—SO₂-aryl), and thiocarbonyl (═S); and thehydrocarbyl moieties C₁-C₂₀ alkyl (preferably C₁-C₁₈ alkyl, morepreferably C₁-C₁₂ alkyl, most preferably C₁-C₆ alkyl), C₂-C₂₀ alkenyl(preferably C₂-C₁₈ alkenyl, more preferably C₂-C₁₂ alkenyl, mostpreferably C₂-C₆ alkenyl), C₂-C₂₀ alkynyl (preferably C₂-C₁₈ alkynyl,more preferably C₂-C₁₂ alkynyl, most preferably C₂-C₆ alkynyl), C₅-C₂₀aryl (preferably C₅-C₁₄ aryl), C₆-C₂₄ alkaryl (preferably C₆-C₁₈alkaryl), and C₆-C₂₄ aralkyl (preferably C₆-C₁₈ aralkyl).

In addition, the aforementioned functional groups may, if a particulargroup permits, be further substituted with one or more additionalfunctional groups or with one or more hydrocarbyl moieties such as thosespecifically enumerated above. Analogously, the above-mentionedhydrocarbyl moieties may be further substituted with one or morefunctional groups or additional hydrocarbyl moieties such as thosespecifically enumerated.

By “substantially free of” a particular type of chemical compound ismeant that a composition or product contains less 10 wt. % of thatchemical compound, preferably less than 5 wt. %, more preferably lessthan 1 wt. %, and most preferably less than 0.1 wt. %. For instance, thedepolymerization product herein is “substantially free of” metalcontaminants, including metals per se, metal salts, metallic complexes,metal alloys, and organometallic compounds.

“Optional” or “optionally” means that the subsequently describedcircumstance may or may not occur, so that the description includesinstances where the circumstance occurs and instances where it does not.For example, the phrase “optionally substituted” means that anon-hydrogen substituent may or may not be present on a given atom, and,thus, the description includes structures wherein a non-hydrogensubstituent is present and structures wherein a non-hydrogen substituentis not present.

Accordingly, the invention features a method for depolymerizing apolymer having a backbone containing electrophilic linkages. Theelectrophilic linkages may be, for example, ester linkages (—(CO)—O—),carbonate linkages (—O—(CO)—O)—, urethane linkages (—O—(CO)—NH),substituted urethane linkages (—O—(CO)—NR—, where R is a nonhydrogensubstituent such as alkyl, aryl, alkaryl, or the like), amido linkages(—(CO)—NH—), substituted amido linkages (—(CO)—NR— where R is as definedpreviously, thioester linkages (—(CO)—S—), sulfonic ester linkages(—S(O)₂—O—), and the like. Other electrophilic linkages that can becleaved using nucleophilic reagents will be known to those of ordinaryskill in the art of organic chemistry and polymer science and/or can bereadily found by reference to the pertinent texts and literature. Thepolymer undergoing depolymerization may be linear or branched, and maybe a homopolymer or copolymer, the latter including random, block,multiblock, and alternating copolymers, terpolymers, and the like.Examples of polymers that can be depolymerized using the methodology ofthe invention include, without limitation:

poly(alkylene terephthalates) such as fiber-grade PET (a homopolymermade from monoethylene glycol and terephthalic acid), bottle-grade PET(a copolymer made based on monoethylene glycol, terephthalic acid, andother comonomers such as isophthalic acid, cyclohexene dimethanol,etc.), poly (butylene terephthalate) (PBT), and poly(hexamethyleneterephthalate);

poly(alkylene adipates) such as poly(ethylene adipate),poly(1,4-butylene adipate), and poly(hexamethylene adipate);

poly(alkylene suberates) such as poly(ethylene suberate);

poly(alkylene sebacates) such as poly(ethylene sebacate);

poly(ε-caprolactone) and poly(β-propiolactone);

poly(alkylene isophthalates) such as poly(ethylene isophthalate);

poly(alkylene 2,6-naphthalene-dicarboxylates) such as poly(ethylene2,6-naphthalene-dicarboxylate);

poly(alkylene sulfonyl-4,4′-dibenzoates) such as poly(ethylenesulfonyl-4,4′-dibenzoate);

poly(p-phenylene alkylene dicarboxylates) such as poly(p-phenyleneethylene dicarboxylates);

poly(trans-1,4-cyclohexanediyl alkylene dicarboxylates) such aspoly(trans-1,4-cyclohexanediyl ethylene dicarboxylate);

poly(1,4-cyclohexane-dimethylene alkylene dicarboxylates) such aspoly(1,4-cyclohexane-dimethylene ethylene dicarboxylate);

poly([2.2.2]-bicyclooctane-1,4-dimethylene alkylene dicarboxylates) suchas poly([2.2.2]-bicyclooctane-1,4-dimethylene ethylene dicarboxylate);

lactic acid polymers and copolymers such as (S)-polylactide,(R,S)-polylactide, poly(tetramethylglycolide), andpoly(lactide-co-glycolide); and

polycarbonates of bisphenol A, 3,3′-dimethylbisphenol A,3,3′,5,5′-tetrachlorobisphenol A, 3,3′,5,5′-tetramethylbisphenol A;

polyamides such as poly(p-phenylene terephthalamide) (Kevlar®);

poly(alkylene carbonates) such as poly(propylene carbonate);

polyurethanes such as those available under the tradenames Baytec® andBayfil®, from Bayer Corporation; and

polyurethane/polyester copolymers such as that available under thetradename Baydar®, from Bayer Corporation.

Depolymerization of the polymer is carried out, as indicated above, inthe presence of a nucleophilic reagent and a catalyst. Nucleophilicreagents, as will be appreciated by those of ordinary skill in the art,include monohydric alcohols, diols, polyols, thiols, primary amines, andthe like, and may contain a single nucleophilic moiety or two or morenucleophilic moieties, e.g., hydroxyl, sulfhydryl, and/or amino groups.The nucleophilic reagent is selected to correspond to the particularelectrophilic linkages in the polymer backbone, such that nucleophilicattack at the electrophilic linkage results in cleavage of the linkage.For example, a polyester can be cleaved at the ester linkages within thepolymer backbone using an alcohol, preferably a primary alcohol, mostpreferably a C₂-C₄ monohydric alcohol such as ethanol, isopropanol, andt-butyl alcohol. It will be appreciated that such a reaction cleaves theester linkages via a transesterification reaction, as will beillustrated infra.

The preferred catalysts for the depolymerization reaction are carbenesand carbene precursors. Carbenes include, for instance, diarylcarbenes,cyclic diaminocarbenes, imidazol-2-ylidenes, 1,2,4-triazol-3-ylidenes,1,3-thiazol-2-ylidenes, acyclic diaminocarbenes, acyclicaminooxycarbenes, acyclic aminothiocarbenes, cyclic diborylcarbenes,acyclic diborylcarbenes, phosphinosilylcarbenes,phosphinophosphoniocarbenes, sulfenyl-trifluoromethylcarbene, andsulfenyl-pentafluorothiocarbene. See Bourissou et al. (2000), citedsupra. Preferred carbenes are heteroatom-stabilized carbenes andpreferred carbene precursors are precursors to heteroatom-stabilizedcarbenes. nitrogen-containing carbenes, with N-heterocyclic carbenesmost preferred.

In one embodiment, heteroatom-stabilized carbenes suitable asdepolymerization catalysts herein have the structure of formula (I)

wherein the various substituents are as follows:

E¹ and E² are independently selected from N, NR^(E), O, P, PR^(E), andS, R^(E) is hydrogen, heteroalkyl, or heteroaryl, and x and y areindependently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively. When E¹ and E² are other thanO or S, then E¹ and E² may be linked through a linking moiety thatprovides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms. In the latter case, the heterocyclic ring may be aliphaticor aromatic, and may contain substituents and/or heteroatoms. Generally,such a cyclic group will contain 5 or 6 ring atoms.

For example, in representative compounds of formula (I):

-   -   (1) E¹ is O or S and x is 1;    -   (2) E¹ is N, x is 1, and E¹ is linked to E²;    -   (3) E¹ is N, x is 2, and E¹ and E² are not linked;    -   (4) E¹ is NR^(E), x is 1, and E¹ and E² are not linked; or    -   (5) E¹ is NR^(E), x is zero, and E¹ is linked to E².

R¹ and R² are independently selected from branched C₃-C₃₀ hydrocarbyl,substituted branched C₃-C₃₀ hydrocarbyl, heteroatom-containing branchedC₄-C₃₀ hydrocarbyl, substituted heteroatom-containing branched C₄-C₃₀hydrocarbyl, cyclic C₅-C₃₀ hydrocarbyl, substituted cyclic C₅-C₃₀hydrocarbyl, heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, andsubstituted heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl. Preferably,at least one of R¹ and R², and more preferably both R¹ and R², arerelatively bulky groups, particularly branched alkyl (includingsubstituted and/or heteroatom-containing alkyl), aryl (includingsubstituted aryl, heteroaryl, and substituted heteroaryl), alkaryl(including substituted and/or heteroatom-containing aralkyl), andalicyclic. Using such sterically bulky groups to protect the highlyreactive carbene center has been found to kinetically stabilize singletcarbenes, which are preferred reaction catalysts herein. Particularsterically bulky groups that are suitable as R¹ and R² are optionallysubstituted and/or heteroatom-containing C₃-C₁₂ alkyl, tertiary C₄-C₁₂alkyl, C₅-C₁₂ aryl, C₆-C₁₈ alkaryl, and C₅-C₁₂ alicyclic, with C₅-C₁₂aryl and C₆-C₁₂ alkaryl particularly preferred. The latter substituentsare exemplified by phenyl optionally substituted with 1 to 3substituents selected from lower alkyl, lower alkoxy, and halogen, andthus include, for example, p-methylphenyl, 2,6-dimethylphenyl, and2,4,6-trimethylphenyl (mesityl).

L¹ and L² are linkers containing 1 to 6 spacer atoms, and areindependently selected from heteroatoms, substituted heteroatoms,hydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, and substituted heteroatom-containing hydrocarbylene;and m and n are independently zero or 1, meaning that each of L¹ and L²is optional. Preferred L¹ and L² moieties include, by way of example,alkylene, alkenylene, arylene, aralkylene, any of which may beheteroatom-containing and/or substituted, or L¹ and/or L² may be aheteroatom such as O or S, or a substituted heteroatom such as NH, NR(where R is alkyl, aryl, other hydrocarbyl, etc.), or PR; and

In one preferred embodiment, E¹ and E² are independently N or NR^(E) andare not linked, such that the carbene is an N-heteroacyclic carbene. Inanother preferred embodiment, E¹ and E² are N, x and y are 1, and E¹ andE² are linked through a linking moiety such that the carbene is anN-heterocyclic carbene. N-heterocyclic carbenes suitable herein include,without limitation, compounds having the structure of formula (II)

wherein R¹, R², L¹, L², m, and n are as defined above for carbenes offormula (I). In carbenes of structural formula (II), L is ahydrocarbylene, substituted hydrocarbylene, heteroatom-containinghydrocarbylene, or substituted heteroatom-containing hydrocarbylenelinker, wherein two or more substituents on adjacent atoms within L maybe linked to form an additional cyclic group. L is a hydrocarbylene,substituted hydrocarbylene, heteroatom-containing hydrocarbylene, orsubstituted heteroatom-containing hydrocarbylene linker, wherein two ormore substituents on adjacent atoms within L may be linked to form anadditional cyclic group. For example, L may be —CR³R⁴—CR⁵R⁶— or—CR³═CR⁵—, wherein R³, R⁴, R⁵, and R⁶ are independently selected fromhydrogen, halogen, C₁-C₁₂ alkyl, or wherein any two of R³, R⁴, R⁵, andR⁶ may be linked together to form a substituted or unsubstituted,saturated or unsaturated ring.

Accordingly, when L is —CR³R⁴—CR⁵R⁶— or —CR³═CR⁵—, the carbene has thestructure of formula (II)

in which q is an optional double bond, s is zero or 1, and t is zero or1, with the proviso that when q is present, s and t are zero, and when qis absent, s and t are 1.

Certain carbenes are new chemical compounds and are claimed as suchherein. These are compounds having the structure of formula (I) whereina heteroatom is directly bound to E¹ and/or E². e.g., with the provisothat a heteroatom is directly bound to E¹, E², or to both E¹ and E², andwherein the carbene may be in the form of a salt (such that it ispositively charged and associated with a negatively charged counterion).These novel carbenes are those wherein a heteroatom is directly bound toE¹ and/or E², and include, solely by way of example, carbenes of formula(I) wherein E¹ and/or E² is NR^(E) and R^(E) is a heteroalkyl orheteroaryl group such as an alkoxy, alkylthio, aryloxy, arylthio,aralkoxy, or aralkylthio moiety. Other such carbenes are those wherein xand/or y is at least 1, and L¹ and/or L² is heteroalkyl, heteroaryl, orthe like, wherein the heteroatom within L¹ and/or L² is directly boundto E¹ and/or E², respectively.

Representative of such novel carbenes are compounds of formula (I)wherein E¹ is NR^(E), and R^(E) is alkoxy, substituted alkoxy, aryloxy,substituted aryloxy, aralkoxy, or substituted aralkoxy. A preferredsubset of such carbenes are those wherein E² is N, x is zero, y is 1,and E¹ and E² are linked through a substituted or unsubstituted loweralkylene or lower alkenylene linkage. A more preferred subset of suchcarbenes are those wherein R^(E) is lower alkoxy or monocyclicaryl-substituted lower alkoxy, E¹ and E² are linked through a moiety—CR³R⁴—CR⁵R⁶or —CR³═CR⁵—, wherein R³, R⁴, R⁵, and R⁶ are independentlyselected from hydrogen, halogen, and C₁-C₁₂ alkyl, n is 1, L² is loweralkylene, and R² is monocyclic aryl or substituted monocyclic aryl.Examples 8-11 describe syntheses of representative compounds within thisgroup.

As indicated previously, suitable catalysts for the presentdepolymerization reaction are also precursors to carbenes, preferablyprecursors to N-heterocyclic and N-heteroacyclic carbenes. In oneembodiment, the precursor is a tri-substituted methane compound havingthe structure of formula (PI)

wherein E¹, E², x, y, R¹, R², L¹, L², m, and n are as defined forcarbenes of structural formula (I), and wherein R⁷ is selected fromalkyl, heteroalkyl, aryl, heteroaryl, aralkyl, or heteroaralkyl, and issubstituted with at least one electron-withdrawing substituent such asfluoro, fluoroalkyl (including perfluoroalkyl), chloro, nitro, acytyl.It will be appreciated that the foregoing list is not exhaustive andthat any electron-withdrawing group may serve as a substituent providingthat the group does not cause unwanted interaction of the catalyst withother components of the depolymerization mixture or adversely affect thedepolymerization reaction in any way. Specific examples of R⁷ groupsthus include p-nitrophenyl, 2,4-dinitrobenzyl, 1,1,2,2-tetrafluoroethyl,pentafluorophenyl, and the like.

Catalysts of formula (PI) are new chemical entities. Representativesyntheses of such compounds are described in Examples 13 and 14 herein.As may be deduced from those examples, compounds of formula (PI) whereinE¹ and E² are N may be synthesized from the corresponding diamine and anappropriately substituted aldehyde.

Another carbene precursor useful as a catalyst in the presentdepolymerization reaction has the structure of formula (PII)

wherein E¹, E², x, y, R¹, R², L¹, L², m, and n are as defined forcarbenes of structural formula (I), M is a metal, e.g., gold, silver,other main group metals, or transition metals, with Ag, Cu, Ni, Co, andFe generally preferred, and Ln is a ligand, generally an anionic orneutral ligand that may or may not be the same as -E¹-[(L¹)_(m)-R¹]_(x)or -E²-[(L²)_(n)-R²]_(y). Generally, carbene precursors of formula (PII)can be synthesized from a carbene salt and a metal oxide; see, e.g., thesynthesis described in detail in Example 12.

Still another carbene precursor suitable as a depolymerization catalystherein is a tetrasubstituted olefin having the structure of formula(PIII)

wherein: E¹, E², x, y, R¹, R², L¹, L², m, and n are defined as forcarbenes of structural formula (I); E³ and E⁴ are defined as for E¹ andE²; v and w are defined as for x and y; R⁸ and R⁹ are defined as for R¹and R²; L³ and L⁴ are defined as for L¹ and L²; and h and k are definedas for m and n. These olefins are readily formed from N,N-diaryl- andN,N-dialkyl-N-heterocyclic carbene salts and a strong base, typically aninorganic base such as a metal alkoxide.

As with the carbenes per se, those catalyst precursors having thestructure of formula (PII) or (PIII) in which a heteroatom is directlybound to an “E” moiety, i.e., to E¹, E², E³, and/or E⁴, are new chemicalentities. Preferred such precursors are those wherein the “E” moietiesare NR^(E) or linked N atoms, and the directly bound heteroatom withinR^(E) is oxygen or sulfur.

The depolymerization reaction may be carried out in an inert atmosphereby dissolving a catalytically effective amount of the selected catalystin a solvent, combining the polymer and the catalyst solution, and thenadding the nucleophilic reagent. In a particularly preferred embodiment,however, the polymer, the nucleophilic reagent, and the catalyst (e.g.,a carbene or a carbene precursor) are combined and dissolved in asuitable solvent, and depolymerization thus occurs in a one-stepreaction.

Preferably, the reaction mixture is agitated (e.g., stirred), and theprogress of the reaction can be monitored by standard techniques,although visual inspection is generally sufficient, insofar as atransparent reaction mixture indicates that the polymer has degraded toan extent sufficient to allow all degradation products to go intosolution. Examples of solvents that may be used in the polymerizationreaction include organic, protic, or aqueous solvents that are inertunder the depolymerization conditions, such as aromatic hydrocarbons,chlorinated hydrocarbons, ethers, aliphatic hydrocarbons, or mixturesthereof. Preferred solvents include toluene, methylene chloride,tetrahydrofuran, methyl t-butyl ether, Isopar, gasoline, and mixturesthereof. Supercritical fluids may also be used as solvents, with carbondioxide representing one such solvent. Reaction temperatures are in therange of about 0° C. to about 100° C., typically at most 80° C.,preferably 60° C. or lower, and most preferably 30° C. or less, and thereaction time will generally be in the range of about 12 to 24 hours.Pressures range from atmospheric to pressures typically used inconjunction with supercritical fluids, with the preferred pressure beingatmospheric.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples that follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

All patents, patent applications, and publications mentioned herein arehereby incorporated by reference in their entireties.

EXPERIMENTAL

General Procedures. ¹H and ¹³C NMR spectra were recorded on aBruke-Avance (400 MHz for ¹H and 100 MHz for ¹³C). All NMR spectra wererecorded in CDCl₃. Materials. Solvents were obtained from Sigma-Aldrichand purified by distillation. Other reagents were obtained commerciallyor synthesized as follows: poly(propylene carbonate), poly(bisphenol Acarbonate), poly(1,4-butylene adipate), 1-ethyl-3-methyl-1-H-imidazoliumchloride, ethylene glycol, butane-2,3-dione monooxime, ammoniumhexafluorophosphate, pentafluorobenzaldehyde, and mesityl diamine,obtained from Sigma-Aldrich;1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene, synthesized according tothe method of Arduengo et al. (1999) Tetrahedron 55:14523; N,N-diphenylimidazoline, chloride salt, synthesized according to the method ofWanzlick et al. (1961) Angew. Chem. 73:493 and Wanzlick et al. (1962)Angew. Chem. 74:128, and Wanzlick et al. (1963) Chem. Ber. 96:3024;1,3,5-tribenzyl-[1,3,5]triazinane, synthesized according to the methodof Arduengo et al. (1992) J. Am. Chem. Soc. 114:5530, cited supra.

Example 1 Depolymerization of Poly(propylene carbonate) (M_(w)=50,000)with isolated carbene

7 mg (0.02 mmol) of 1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidenedissolved in toluene (0.6 mL), was added to a stirred mixture of 0.5 gof poly(propylene carbonate) in toluene (10 mL), under N₂. Afterstirring for 5 minutes at room temperature, 2 mL of methanol were addedto the reaction mixture and the temperature was brought to 80° C.Stirring was continued for 3 hours followed by the evaporation of thesolvent in vacuo. The ¹H and ¹³C NMR spectra showed the presence of asingle monomer, 4-methyl-[1,3]-dioxolan-2-one. However, there were 4peaks in the GC-MS. GC-MS: a) m/z (5%) 5.099 min=106 (42), 103 (5), 91(100), 77 (8), 65 (8), 51 (8) b) m/z (5%) 5.219 min=106 (60), 105 (30),103 (8), 91 (100), 77 (8), 65 (5), 51 (5) c) m/z (85%) 6.750 min=102(15), 87 (40), 58 (20), 57 (100). Major product. d) m/z (5%) 9.030min=136 (10), 135 (100), 134 (70), 120 (85), 117 (8), 103 (5), 91 (14),77 (10), 65 (5). ¹H NMR:1.4 (d, 3H), 3.9 (t, 1H), 4.5 (t, 1H), 4.8 (m,1H). ¹³C NMR: 18.96, 70.42, 73.43, 154.88

Example 2 Depolymerization of Poly(Bisphenol A carbonate) (M_(w)=65,000)with isolated carbene

7 mg (0.02 mmol) of 1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidinedissolved in toluene (1 mL), was added to a stirred mixture of 0.5 g ofpoly(bisphenol A carbonate) in toluene (10 mL), under N₂. After stirringfor 5 minutes at room temperature, 2 mL of methanol were added to thereaction mixture. The temperature was brought to 80° C. and stirring wascontinued for 18 hours followed by the evaporation of the solvent invacuo. The ¹H and ¹³C NMR spectra showed the presence of two compoundsidentified as, bisphenol A and carbonic acid4-[1-hydroxy-phenyl)-1-methyl-ethyl]-phenyl ester4-[1-(4-methoxy-phenyl)-1-methyl-ethyl]phenyl ester. However, GC-MSindicated 4 peaks. GC-MS: a) m/z (5%) 5.107 min=106 (40), 103 (5), 91(100), 77 (8), 65 (8), 51 (8) b) m/z (5%) 5.210 min=106 (60), 105 (30),103 (8), 91 (100), 77 (8), 65 (5), 51 (5) c) m/z (60%) 14.301 min=228(30), 213 (100), 119 (15), 91 (10). Major product d) m/z (30%) 16.016min=495 (30), 333 (10), 319 (20), 299 (5), 281 (5), 259 (25), 239 (38),197 (40), 181 (12), 151 (12), 135 (100), 119 (10), 91 (10). ¹H NMR:1.6-1.8 (m), 2,4 (s), 3.96 (s), 6.7-6.8 (t), 7.0-7.3 (m).

Example 3 Depolymerization of Poly(1,4-butylene adipate) (M_(w)=12,000)with isolated carbene

0.006 g (0.02 mmol) of 1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidinedissolved in toluene (1 mL), was added to a stirred mixture of 1.0 g ofpoly(1,4-butylene adipate) in toluene (10 mL), under N₂. After stirringfor 5 minutes at room temperature, 2 mL of methanol were added to thereaction mixture. The temperature was brought to 80° C. and stirring wascontinued for 6 hours followed by the evaporation of the solvent invacuo. The ¹H and ¹³C NMR showed the presence of a single product, andthe GC-MS showed two products. GC-MS: a) m/z (95%) 5.099 min=143 (80),142 (20), 115 (20), 114 (100), 111 (70), 101 (65), 87 (12), 83 (25), 82(12), 74 (36), 73 (26), 69 (10), 59 (72), 55 (60). Major product. b) m/z(5%) 12.199 min=201 (4), 161 (6), 143 (100), 129 (32), 116 (12), 115(25), 111 (70), 101 (12), 87 (10), 83 (15), 73 (34), 71 (12), 59 (14),55 (42). ¹H NMR: 1.67 (m), 2.32 (s), 4.08 (s). ¹³C NMR: 24.26, 25.18,33.74, 63.75, 173.23

Example 4 Depolymerization of Poly(propylene carbonate) (M_(w)=50,000)with in-situ carbene

To a mixture of 7 mg (0.047 mmol) of 1-ethyl-3-methyl-1-H-imidazoliumchloride in tetrahydrofuran (THF) was added 4 mg (0.038 mmol) ofpotassium t-butoxide (t-BOK), under N₂. After 30 min stirring, 0.1 mL ofthe reaction mixture was transferred to a flask that was charged with0.5 g of poly(propylene carbonate) in 10 mL of THF. The reaction mixturewas stirred for 10 min at room temperature followed by the addition of 2mL of methanol. Stirring was continued at room temperature for 3 hours.Solvent was removed and the ¹H and ¹³C NMR spectra showed the presenceof a single product, 4-methyl-[1,3]-dioxolan-2-one. However, before theremoval of the solvent the GC-MS of the crude reaction mixture showed 6different compounds. GC-MS: a) m/z (15%) 6.268 min=119 (4), 90 (100), 75(4), 59 (25). b) m/z (5%) 6.451 min=104 (40), 103 (30), 90 (5), 77 (5),59 (100), 58 (10), 57 (10). c) m/z (70%) 6.879 min=102 (10), 87 (25), 58(14), 57 (100). Major product. d) m/z (1%) 7.565 min=103 (40), 89 (5),59 (100), 58 (5), 57 (8). e) m/z (4%) 8.502 min=207 (14), 133 (10), 103(35), 90 (10), 89 (10), 59 (100), 58 (12), 57 (14). f) m/z (5%) 8.936min=148 (8), 118 (8), 117 (15), 103 (20), 77 (60), 72 (8), 59 (100), 58(5), 57 (5). ¹H NMR:1.4 (d, 3H), 3.9 (t, 1H), 4.5 (t, 1H), 4.8 (m, 1H).¹³C NMR: 18.96, 70.42, 73.43, 154.88

Example 5 Depolymerization of Poly(bisphenol A carbonate) (M_(w)=65,000)with in situ carbene

To a mixture of 7 mg (0.047 mmol) of 1-ethyl-3-methyl-1-H-imidazoliumchloride in THF (1 mL) was added 4 mg (0.038 mmol) of t-BOK, under N₂.After 30 min, stirring 0.1 mL of the reaction mixture was transferred toa flask that was charged with 0.5 g of poly(bisphenol A carbonate) in 10mL of THF. The reaction mixture was stirred for 10 min at roomtemperature followed by the addition of 2 mL of methanol. Stirring wascontinued at room temperature for 3 hours. The solvent was removed invacuo and the ¹H, ¹³C NMR and GC-MS spectra showed a mixture of monomerand oligomers, where the major product was bisphenol A. GC-MS: a) m/z(10%) 12.754 min=212 (30), 198 (20), 197 (100), 182 (10), 181 (10), 179(10), 178 (10), 165 (8), 152 (8), 135 (10), 119 (12), 103 (15), 91 (12),77 (10), 65 (5). b) m/z (5%) 13.674 min=282 (5), 281 (10), 255 (8), 229(10), 228 (40), 214 (20), 213 (100), 208 (30), 197 (30), 191 (5), 181(5), 179 (5), 165 (10), 152 (8), 135 (25), 134 (25), 133 (5), 120 (5),119 (50), 115 (10), 103 (10), 99 (5), 97 (5), 96 (5), 91 (30), 79 (5),77 (10), 65 (8). c) m/z (35%) 14.286 min=228 (34), 214 (20), 213 (100),197 (5), 165 (5), 135 (5), 119 (20), 107 (5), 91 (10), 77 (5), 65 (5).Major Product. d) m/z (35%) 15.189 min=286 (20), 272 (15), 271 (100),227 (5), 212 (5), 197 (3), 183 (2), 169 (3), 133 (3), 119 (5). e) m/z(10%) 15.983 min=344 (20), 330 (20), 329 (100), 285 (5), 269 (3), 226(3), 211 (2), 183 (3), 165 (3), 153 (2), 133 (6), 121 (2), 91 (2), 77(1), 59 (3).

Example 6

for 15 minutes. Ethylene glycol (2.3 g) and PET (0.25 g) (pelletsobtained from Aldrich dissolved in CHCl₃ and trifluoroacetic acid andprecipitated with methanol to form a white powder) were combined to forma PET slurry. The catalyst was added to the slurry with approximately 5additional mL THF. After 2 hours, the solution became more transparent,indicating dissolution of the components of the depolymerizationmixture. The admixture was stirred overnight, yielding a completelyclear solution the following day. the THF was removed, yielding 225 mgof white solid. ¹H NMR¹³C NMR, and GC-MS were all consistent withbis(hydroxy ethylene) terephthalate.

Example 7

Depolymerization of PET according to the above scheme: 25 mg of1,3-dimethyl imidazole, iodide salt, and 11 mg of t-BOK were placed in avial with 2 mL of THF and stirred for 15 min. Methanol (3.11 g) and PET(308 mg, as in Example 6) were combined with 5 mL of THF to form aninsoluble mixture. The catalyst mixture was filtered into thePET/methanol mixture. After 1 hour, there was a noticeable increase intransparency. After 14 hours, the solution was completely homogeneousand clear. The solvent was removed by rotary evaporation to yield awhite crystalline product (250 mg). ¹H NMR indicated complete conversionto dimethyl terephthalate.

Examples 6 and 7 may be better understood by reference to the syntheticroute used to prepare the PET and the possible depolymerization productsobtained therefrom. The PET obtained in each example was prepared bysynthesis according to a two-step transesterification process fromdimethyl teraphthalate (DMT) and excess ethylene glycol (EO) in thepresence of a metal alkanoate or acetate of calcium, zinc, manganese,titanium etc. The first step generates bis(hydroxy ethylene)teraphthalate (BHET) with the elimination of methanol and the excess EO.The BHET is heated, generally in the presence of a transesterificationcatalyst, to generate high polymer. This process is generallyaccomplished in a vented extruder to remove the polycondensate (EO) andgenerate the desired thermoformed object from a low viscosity precursor.The reaction takes place according to the following scheme:

The different options for chemical recycling are regeneration of thebase monomers (DMT and EG), glycolysis of PET back to BHET,decomposition of PET with propylene glycol and reaction of thedegradation product with maleic anhydride to form “unsaturatedpolyesters” for fiber reinforced composites and decomposition withglycols, followed by reaction with dicarboxylic acids to produce polyolsfor urethane foam and elastomers.

In Example 7, PET powder was slurried in a THF/methanol solvent mixture.N-heterocyclic carbene (3-5 mol %), generated in situ, was added andwithin approximately 3 hours the PET went into solution. Anaylsis of thedegradation product indicated quantitative consumption of PET anddepolymerization via transesterification to EO and DMT. The DMT isreadily recovered by recrystallization, while EO can be recovered bydistillation (FIG. 1). Alternatively, and as established in Example 6,if EO is used as the alcohol (˜50 to 200 mol % excess) in the THFslurry, the depolymerization product is BHET, which is the mostdesirable and can be directly recycled via conventional methods to PET(FIG. 2). The N-heterocyclic carbene catalyst platform is extremelypowerful, as the nature of the substituents has a pronounced effect oncatalyst stability and activity towards different substrates.

The PET depolymerization reactions of Examples 6 and 7 are illustratedschematically below.

The following Examples 8-11 describe synthesis of new carbene precursorsas illustrated in the following scheme:

Example 8 3-Benzyl-1-methoxy-4,5-dimethylimidazolium iodide (2)

Methyl iodide (0.5 mL, 7.8 mmol) was added via syringe to a solution ofimidazole-N-oxide 1 (1.0 g, 4.9 mmol) in ca. 20 mL of CHCl₃ (compound 1was prepared from 1,3,5-tribenzyl-[1,3,5]triazinane and butane-2,3-dionemonooxime using the procedure of Arduengo et al. (1992), supra.) Theresulting mixture was stirred at room temperature overnight. Removal ofthe volatiles in vacuo afforded a thick yellow oil of suitable purity inan undetermined yield. ¹H-NMR (δ, CDCl₃): 10.32 (s, 1H, N—CH—N); 7.39(m, 5H, C₆H₅); 5.56 (s, 2H, NCH₂); 4.38 (s, 3H, OCH₃); 2.27 (s, 3H,CH₃); 2.20 (s, 3H, CH₃).

Example 9 3-Benzyl-1-methoxy-4,5-dimethylimidazolium hexafluorophosphate(3)

Crude iodide 2 was taken up in deionized (DI) water, which separated theproduct from small amounts of a dark, insoluble residue. The watersolution was decanted to a second flask and a solution of ammoniumhexafluorophosphate (950 mg, ca. 5.8 mmol) in 10 mL of DI water wasadded in portions. An oil separated during the addition, and thesupernatant solution was decanted out. The oil was crushed in cold (0°C.), and subsequently recrystallized in methanol. Yield: 1.3 g (73% from1). ¹H-NMR (δ, CDCl₃): 8.67 (s, 1H, N—CH—N); 7.39 (m, 3H, C₆H₅); 7.29(d, 2H, C₆H₅); 5.24 (s, 2H, NCH₂); 4.21 (s, 3H, OCH₃); 2.27 (s, 3H,CH₃); 2.17 (s, 3H, CH₃).

Example 10 1-Benzyloxy-3-benzyl-4,5-dimethylimidazolium bromide (4)

Benzyl bromide (1.2 mL, ca. 10 mmol) was added via syringe to arefluxing suspension of 1 (1.0 g, 5.0 mmol) in dry benzene. A darkorange oil separated after refluxing for 6 h, and cooling to roomtemperature. The supernatant was decanted and the remaining oil wasdried under vacuum overnight, which caused the product to solidify. Thesolid mass was crushed in pentane, filtered and dried under vacuum.Yield: 1.34 g (63%). ¹H-NMR (δ, CDCl₃): 11.04 (s, 1H, N—CH—N); 7.6-7.2(ov. m, 10H, 2×CrH₅); 5.59, 5.58 (s+s, N—CH₂, O—CH₂); 2.09, 1.94 (s, 3H,CH₃, CH₃). ³C-NMR (δ, CDCl₃): 132.8 (OCH₂-^(i)C₆H₅); 132.5 (NCN); 131.5(NCH₂-^(i)C₆H₅); 130.6, 130.3, 129.2, 129.0, 129.0, 128.9, 128.0(^(omp)C₆H₅); 124.8; 124.1 (NCCN; 83.9 (OCH₂); 51.2 (NCH₂); 8.89 (CH₃);7.11 (CH₃).

Example 11 3-Benzyl-1-benzyloxy-4,5-dimethylimidazoliumhexafluorophosphate (5)

A batch of crude bromide 4 (still as an oil before drying under vacuum)was dissolved in DI water and extracted with hexanes. The aqueous layerwas separated and a solution of ammonium hexafluorophosphate (ca. 1.3equiv.) was added dropwise with constant stirring. The yellow oildeposited on the walls of the flask was dissolved in warm methanol and afew drops of hexanes were added. Cooling to room temperature affordedoff-white crystals of pure 5, which were rinsed with pentane and driedunder vacuum. Yield: (82% from 1). ¹H-NMR (δ, CDCl₃): 8.42 (s, 1H,N—CH—N); 7.45-7.35, 7.18 (ov. m, C₆H₅); 5.31, 5.20 (s+s, N—CH₂, O—CH₂);2.13 (s, 3H, CH₃); 2.05 (s, 3H, CH₃).

Example 12

Bis(1-Benzyloxy-3-benzyl-4,5-dimethylimidazolylidene)silver(I)dibromoargentate (6)

The carbene precursor 6 was prepared as follows: A mixture of silveroxide (128 mg, 0.55 mmol) and imidazolium bromide 4 (396 mg, 1.06 mmol)was taken up in dry CH₂Cl₂ and stirred at room temperature for 90minutes. The dark orange suspension was filtered through a pad of celiteand evaporated to dryness, yielding an orange powder. Crystallizationfrom THF afforded a white powder (2 crops). Yield: 291 mg (57%). ¹H-NMR(δ, CD₂Cl₂): 7.47-7.32 (ov. m, 10H, 2×C₆H₅); 5.23, 5.22 (s+s, NCH₂,OCH₂); 2.01, 1.95 (s, 3H+3H, CH₃, CH₃). ¹³C-NMR (δ, CD₂Cl₂): 136.2(NCN); 133.3 (OCH₂-^(i)C₆H₅); 130.8 (NCH₂-^(i)C₆H₅); 130.7, 130.0;129.3, 129.3, 128.5, 127.1, 123.9 (^(omp)C₆H₅+NCCN); 82.6 (OCH₂); 54.0(NCH₂); 9.4 (CH₃); 7.8 (CH₃). Anal. Found: C, 47.56; H, 4.26; N, 5.79%.Calc. for C₃₈H₄₀Ag₂Br₂N₄O₂: C, 47.53; H, 4.20; N, 5.83%.

Examples 13 and 14 describe preparation of additional carbene precursorsfrom N,N-diaryl-substituted diamines as illustrated in the schemesbelow.

Example 13

Synthesis of carbene precursor 7(2-pentafluorophenyl-1,3-diphenyl-imidazolidine)

200 mg (0.94 mmol, FW=212.12) N,N′-diphenyl-ethane-1,2-diamine wasplaced in a vial and dissolved in 5 mL CH₂Cl₂. A catalytic amount ofp-toluenesulfonic acid and 50 mg of Na₂SO₄ were added, followed by 230mg (0.94 mmol, FW=196.07) of pentafluorobenzaldehyde. The mixture wasstirred for 8 h. The Na₂SO4 was filtered off and solvent was removedunder reduced pressure to yield a light brown powder 395 mg (FW=436.2),96% yield. ¹H NMR: (400 MHz, CDCl₃, 25° C.) δ=3.7-3.9 (m, 2H), 3.9-4.1(m, 2H), 6.5 (s, 1H), 6.7-6.8 (m, 2H), 6.8-6.9 (m, 1H), 7.2-7.5 (m, 2H).¹⁹F NMR: δ=−143.2 (s br, 2F), −153.7-−153.8 (m, 1F), 161.7-−161.8 (m,2F).

Example 14

Synthesis of carbene precursor 8(2-pentafluorophenyl-1,3-bis-(2,4,6-trimethyl-phenyl)-imidazolidine)

Mesityldiamine (512 mg, 1.7 mmol) was placed into a vial, equipped witha stirbar, with pentafluorobenzaldehyde (340 mg, 1.7 mmol). Glacialacetic acid (5 mL) was added and the reaction was stirred at roomtemperature for 24 h. The acetic acid was removed under reduced pressureand the product was washed several times with cold methanol to affordthe product as a white crystalline solid (543 mg, 65%). ¹H NMR: (400MHz, CDCl₃, 25° C.) δ: 2.2 (s, 12H), 2.3 (s, 6H), 3.5-3.6 (m, 2H),3.9-3.4 (m, 2H), 6.4 (s, 1H), 6.9 (s, 4H). ¹⁹F NMR: −136.3-−136.4 (m,1F), −148.6-−148.7 (m, 1F), −155.8-−155.9 (m, 1F), −163.0-−163.3 (m,2F).

1. A carbene precursor having the structure of formula (PI)

wherein: E¹ and E² are independently selected from N, NR^(E), O, P,PR^(E), and S, R^(E) is hydrogen, heteroalkyl, or heteroaryl, x and yare independently zero, 1, or 2, and are selected to correspond to thevalence state of E¹ and E², respectively, and wherein when E¹ and E² areother than 0 or S, then E¹ and E² may be linked through a linking moietythat provides a heterocyclic ring in which E¹ and E² are incorporated asheteroatoms; R¹ and R² are independently selected from branched C₃-C₃₀hydrocarbyl, substituted branched C₃-C₃₀ hydrocarbyl,heteroatom-containing branched C₄-C₃₀ hydrocarbyl, substitutedheteroatom-containing branched C₄-C₃₀ hydrocarbyl, cyclic C₅-C₃₀hydrocarbyl, substituted cyclic C₅-C₃₀ hydrocarbyl,heteroatom-containing cyclic C₁-C₃₀ hydrocarbyl, and substitutedheteroatom-containing cyclic C₁-C₃₀ hydrocarbyl; L¹ and L² are linkerscontaining 1 to 6 spacer atoms, and are independently selected fromheteroatoms, substituted heteroatoms, hydrocarbylene, substitutedhydrocarbylene, heteroatom-containing hydrocarbylene, and substitutedheteroatom-containing hydrocarbylene; m and n are independently zero or1; and R⁷ is selected from heteroalkyl, aryl, heteroaryl, aralkyl, orheteroaralkyl, substituted with at least one electron-withdrawingsubstituent, with the proviso that, when R⁷ is heteroalkyl, then the atleast one electron-withdrawing substituent is selected from the groupconsisting of fluoro, fluoroalkyl, chloro, nitro, and acytyl.
 2. Thecarbene precursor of claim 1, wherein the at least oneelectron-withdrawing substituent is selected from the group consistingof fluoro, fluoroalkyl (including perfluoroalkyl), chloro, and nitro,acytyl.
 3. The carbene precursor of claim 1, wherein the carbeneprecursor is in the form of a salt.
 4. The carbene precursor of claim 3,wherein the salt is positively charged.
 5. The carbene precursor ofclaim 4, wherein the salt is associated with an anionic counterion. 6.The carbene precursor of claim 5, wherein the anionic counterion isselected from the group consisting of a halide ion, ahexafluorophosphate anion, and the like.
 7. The carbene precursor ofclaim 6, wherein the halide ion is selected from the group consisting ofI, Br, and Cl.